1. Field of the Invention
The invention relates generally to photographic apparatus and more particularly to a focusing mechanism for a lens in a camera or a projector, for example.
2. Description of the Prior Art
Most cameras, except the fixed focus types, have some means of focusing--i.e., of setting the lens at the correct distance from the film to give a sharp image of the subject to be photographed. There are several ways of doing this; any one of them to be efficient must fulfill the following requirements: it must be free from backlash and play, and it must maintain the lens rigidly in a plane parallel with that of the film.
One recently devised focusing mechanism in a camera is disclosed in the patent application cross-referenced above. That mechanism is for a lens mounted for translation bi-directionally along its optical axis between an infinity focus position and a close-up (e.g., 3') focus position, and includes a cam ring having respective positions corresponding to the infinity and close-up positions of the lens. A compression spring urges the lens in a rearward direction along the optical axis, i.e., toward the infinity position, and holds a cam follower extending from the lens against an inclined cam surface on the cam ring. In operation, the cam ring must be rotated bi-directionally about the optical axis in order to translate the lens bi-directionally between its infinity and close-up positions. More specifically, rotation of the cam ring in a forward direction from its position corresponding to the infinity position of the lens to its other corresponding position will move the cam surface to translate the lens in a forward direction from its infinity position to its close-up position. Then, rotation of the cam ring in a reverse direction back to its initial position will permit the compression spring to translate the lens in the rearward direction back to its infinity position. An electromagnetic actuator for rotating the cam ring from its initial position to its other position includes a planar armature fixed to the cam ring for rotation with the cam ring about the optical axis. The armature has a flat coil located in a magnetic gap and is rotated in the forward direction about the optical axis, to rotate the cam ring to its other position, in response to current applied to the coil. Discontinuing current to the coil permits cantilever return springs to rotate the armature in the reverse direction about the optical axis, which rotates the cam ring back to its initial position.
While the focusing mechanism in the referenced patent application may operate satisfactorily, the amount of current applied to the coil for rotating the armature in the forward direction about the optical axis must be great enough to overcome the contrary urging of the cantilever return springs, as well as other forces that act to retard movement of the cam ring and the lens. Moreover, current must be continuously applied to the coil, against the contrary urging of the cantilever return springs, for as long as it is desired to hold the lens in its close-up position. Such an arrangement requires relatively large amounts of current, shortening the expected battery life and thereby necessitating a frequent change of batteries.
Another earlier devised focusing mechanism is disclosed in German Auslegeschrift No. 1101801, published Mar. 9, 1961. That mechanism includes two pawls which are electromagnetically actuated individually by closing separate switches. The two pawls engage respective ratchets, mounted for rotation coaxially with a worm gear. The worm gear is disposed in mesh with an elongate rack located along the barrel housing of a lens. The lens is mounted for translation along an optical axis that extends parallel to the rotational axis of the ratchets and the worm gear. Selective actuation of the pawls will cause the ratchets to rotate in opposite directions, thereby rotating the worm gear in such directions to translate the lens forward or rearward to respective focusing positions. This arrangement is perhaps better suited for focusing a lens in a slide projector, rather than in a camera, because of the considerable amount of space needed for the various components. Moreover, the mechanism requires two electromagnetic means, as well as two pawl and ratchet indexing means, which makes the mechanism relatively complex and expensive.